multifd: Only flush once each full round of memory

multifd: Only flush once each full round of memory

We need to add a new flag to mean to flush at that point.
Notice that we still flush at the end of setup and at the end of
complete stages.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
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Add missing qemu_fflush(), now it passes all tests always.
In the previous version, the check that changes the default value to
false got lost in some rebase.  Get it back.